U.S. patent number 7,109,537 [Application Number 11/068,365] was granted by the patent office on 2006-09-19 for cmos pixel with dual gate pmos.
This patent grant is currently assigned to Dialog Imaging Systems GmbH. Invention is credited to Taner Dosluoglu, Nathaniel Joseph McCaffrey.
United States Patent |
7,109,537 |
Dosluoglu , et al. |
September 19, 2006 |
CMOS pixel with dual gate PMOS
Abstract
A pixel circuit with a dual gate PMOS is formed by forming two
P.sup.+ regions in an N.sup.- well. The N.sup.- well is in a
P.sup.- type substrate. The two P.sup.+ regions form the source and
drain of a PMOS transistor. The PMOS transistors formed within the
N.sup.- well will not affect the collection of the photo-generated
charge as long as the source and drain potentials of the PMOS
transistors are set at a lower potential than the N.sup.- well
potential so that they remain reverse biased with respect to the
N.sup.- well. One of the P.sup.+ regions used to form the source
and drain regions can be used to reset the pixel after it has been
read in preparation for the next cycle of accumulating
photo-generated charge. The N.sup.- well forms a second gate for
the dual gate PMOS transistor since the potential of the N.sup.-
well 12 affects the conductivity of the channel of the PMOS
transistor. The addition of two NMOS transistors enables the
readout signal to be stored at the gate of one of the NMOS
transistors thereby making a snapshot imager possible. The circuit
can be expanded to form two PMOS transistors sharing a common drain
in the N.sup.- well.
Inventors: |
Dosluoglu; Taner (New York,
NY), McCaffrey; Nathaniel Joseph (Stockton, NJ) |
Assignee: |
Dialog Imaging Systems GmbH
(Kirchheim/Teck-Nabern, DE)
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Family
ID: |
32711059 |
Appl.
No.: |
11/068,365 |
Filed: |
February 28, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050156214 A1 |
Jul 21, 2005 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10339190 |
Jan 9, 2003 |
6870209 |
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Current U.S.
Class: |
257/292;
257/E27.131; 257/291; 257/E27.132; 257/290; 257/E31.073;
257/E27.133 |
Current CPC
Class: |
H01L
27/14609 (20130101); H01L 31/112 (20130101); H01L
27/14643 (20130101); H01L 27/14632 (20130101) |
Current International
Class: |
H01L
31/062 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 734 069 |
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Sep 1996 |
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EP |
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0 886 318 |
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Dec 1998 |
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EP |
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2000-253315 |
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Sep 2000 |
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JP |
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Other References
"A 128.times.128-Pixel Standard-CMOS Image Sensor with Electronic
Shutter", by Chye Hunt Aw et al., Solid-State Circuits Conf., 1996,
Digest of Tech. Papers, 42nd Isscc., Feb. 8, 1996, pp. 180-181,
140. cited by other.
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Primary Examiner: Le; Dung A.
Attorney, Agent or Firm: Saile Ackerman LLC Ackerman;
Stephen B. Prescott; Larry J.
Parent Case Text
This is a division of patent application Ser. No. 10/339,190,
filing Jan. 9, 2003, now U.S. Pat. No. 6,870,209, Cmos Pixel With
Dual Gate Pmos, assigned to the same assignee as the present
invention, which is herein incorporated by reference in its
entirety.
Claims
What is claimed is:
1. A pixel circuit, comprising: a P.sup.- silicon substrate; an
N.sup.- well formed in said P.sup.- silicon substrate, wherein said
N.sup.- well formed in said P.sup.- silicon substrate forms a PN
junction which can accumulate signal-generated charge; a first
P.sup.+ region, a second P.sup.+ region, and a third P.sup.+ region
formed in said N.sup.- well; a first PMOS transistor having a
source, a drain, and a channel formed in said N.sup.- well, wherein
said first P.sup.+ region forms said source of said first PMOS
transistor, said second P.sup.+ region forms said drain of said
first PMOS transistor, and that part of said N.sup.- well between
said first P.sup.+ region and said second P.sup.+ region forms said
channel of said first PMOS transistor; a second PMOS transistor
having a source, a drain, and a channel formed in said N.sup.-
well, wherein said third P.sup.+ region forms said source of said
second PMOS transistor, said second P.sup.+ region forms said drain
of said second PMOS transistor, and that part of said N.sup.- well
between said second P.sup.+ region and said third P.sup.+ region
forms said channel of said second PMOS transistor; a first gate
electrode formed over a gate oxide over said channel of said first
PMOS transistor forming the gate of said first PMOS transistor; a
second gate electrode formed over a gate oxide over said channel of
said second PMOS transistor forming the gate of said second PMOS
transistor; a first NMOS transistor having a drain connected to a
first output node, a gate, and a source connected to said source of
said first PMOS transistor; a first N.sup.+ region formed in said
N.sup.- well; a second NMOS transistor having a source connected to
said first N.sup.+ region formed in said N.sup.- well, a drain
connected to said gate of said first NMOS transistor, and a gate
connected to said source of said first NMOS transistor; a third
NMOS transistor having a drain connected to a second output node, a
gate, and a source connected to said source of said second PMOS
transistor; a second N.sup.+ region formed in said N.sup.- well;
and a fourth NMOS transistor having a source connected to said
second N.sup.+ region formed in said N.sup.- well, a drain
connected to said gate of said third NMOS transistor, and a gate
connected to said source of said third NMOS transistor.
2. The pixel circuit of claim 1 wherein said signal-generated
charge is photo-generated charge.
3. The pixel circuit of claim 1 wherein said first P.sup.+ region,
said second P.sup.+, and said third P.sup.+ region are held at a
potential lower than the N- well potential when said junction
between said N.sup.- well and said P.sup.- silicon substrate is
accumulating signal-generated charge.
4. The pixel circuit of claim 1 wherein said junction between said
N.sup.- well and said P.sup.- silicon substrate is reset by
bringing said N.sup.- well to the highest potential in the pixel
circuit less a forward bias potential wherein said forward bias
potential is the potential drop across the junction of said first
P.sup.+ region and said N.sup.- well when said junction is forward
biased.
5. The pixel circuit of claim 1 wherein said gates of said first
PMOS transistor and said second PMOS transistor are held at ground
potential while the potential of said second P.sup.+ region is
raised from ground potential to the highest potential in the pixel
circuit when said junction between said N.sup.- well and said
P.sup.- silicon substrate is being reset.
6. The pixel circuit of claim 1 wherein said gate of said second
PMOS transistor and said second P.sup.+ region are held at the
highest potential in the pixel circuit and said gate of said first
PMOS transistor is held at ground potential for a first time
interval; said second P.sup.+ region is held at ground potential,
said gate of said second PMOS transistor is held at the highest
potential in the pixel circuit, and the potential of said gate of
said first PMOS transistor is ramped from ground potential toward
the highest potential in the pixel circuit during a second time
interval following said first time interval when said junction
between said N.sup.- well and said P.sup.- silicon substrate is
accumulating signal-generated charge.
7. The pixel circuit of claim 1 wherein said gate of said second
PMOS transistor and said second P.sup.+ region are held at ground
potential and a potential proportional to the signal-generated
charge accumulated at said junction between said N.sup.- well and
said P.sup.- silicon substrate is stored at said gate of said third
NMOS transistor when said charge accumulated at said junction
between said N.sup.- well and said P.sup.- silicon substrate is
read.
8. The pixel circuit of claim 1, further comprising: a fifth NMOS
transistor having a source connected to said first output node; a
first sequential row addressing circuit connected to said gate of
said fifth NMOS transistor; a sixth NMOS transistor having a source
connected to said second output node; and a second sequential row
addressing circuit connected to said gate of said sixth NMOS
transistor.
Description
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a CMOS pixel comprising an N.sup.- well
formed in a P.sup.- epitaxial silicon layer with a dual gate PMOS
transistors formed in the N.sup.- well.
(2) Description of the Related Art
U.S. Pat. No. 6,147,362 to Keyser describes a high performance
pixel for active matrix electronic displays. The pixel combines a
compact mesa-isolated PMOS access transistor with a novel, area
efficient high voltage device.
U.S. Pat. No. 6,127,697 to Guidash describes an active pixel sensor
comprising a substrate of a first conductivity type having a
surface containing PMOS and NMOS implants that are indicative of a
sub-micron CMOS process, a photodetector formed at a first depth
from an implant of a second conductivity type that is opposite the
first conductivity type on the surface, and a gate on the surface
adjacent to the photodetector. The photodetector is formed by an
implant of the second conductivity type that is deeper and more
lightly doped than implants used within the sub-micron CMOS
process.
U.S. Pat. No. 5,923,369 to Merrill et al. describes an active pixel
sensor cell array in which a differential amplifier amplifies the
output of each cell. The output of the differential amplifier is
fed back to one of its inputs. The use of the differential
amplifiers reduces fixed pattern noise in the image data generated
by reading the array.
U.S. Pat. No. 5,917,547 to Merrill et al. describes an active pixel
sensor array in which a two stage amplifier amplifies the output of
each cell. The two stage amplifier design reduces fixed pattern
noise in the image data generated by reading the array.
SUMMARY OF THE INVENTION
Active pixel sensors, APS, are of particular value in digital
imaging systems because they can be fabricated using standard CMOS,
complimentary metal oxide semiconductor, processing and because
they have lower power consumption than CCD, charge coupled device,
imagers. As CMOS process parameters shrink, the analog performance
of minimum size transistors deteriorates. It is desirable to have
transistors in the semiconductor well forming the pixel which can
be drawn to a size large enough to improve the analog performance
without impacting the area under which signal-generated carriers,
such as photo-generated carriers, will be generated. This is a
problem using N.sup.+ regions with V.sub.DD bias acting as drains
to form the pixel.
It is a principle objective of this invention to provide a CMOS
pixel circuit formed in an N.sup.- well with a dual gate PMOS, P
channel metal oxide semiconductor, transistor formed in an N.sup.-
well wherein any of the P.sup.+ regions used to form the PMOS
transistor can be used to reset the pixel.
It is another principle objective of this invention to provide a
CMOS pixel circuit formed in an N.sup.- well with a dual gate PMOS
transistor formed in an N.sup.- well with two NMOS, N channel metal
oxide semiconductor, transistors used to read the pixel.
It is another principle objective of this invention to provide a
CMOS pixel circuit formed in an N.sup.- well with two dual gate
PMOS transistors formed in an N.sup.- well with four NMOS
transistors used to read the pixel.
These objectives are achieved by forming an N.sup.- well in a
P.sup.- epitaxial silicon layer. P.sup.+ regions are then formed in
the N.sup.- well to form the source and drain of a PMOS, P channel
metal oxide semiconductor, transistor. The PMOS transistors formed
within the N.sup.- well will not affect the collection of signal
generated carriers as long as the source and drain potentials of
the PMOS transistors are set at a lower potential than the N.sup.-
well potential so that they remain reverse biased with respect to
the N.sup.- well. Typically, but not necessarily, the signal
generated carriers are photo-generated carriers. Any of the P.sup.+
regions used to form the source and drain regions can be used to
reset the pixel after it has been read in preparation for the next
cycle of accumulating signal-generated carriers. The N.sup.- well
forms a second gate for the dual gate PMOS transistor since the
potential of the N.sup.- well 12 affects the conductivity of the
channel of the PMOS transistor.
The drain of the PMOS transistor can be connected to ground
potential and thereby require one less conducting line to operate
each pixel.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a top view of a number of N.sup.- wells formed in a
P.sup.- epitaxial silicon layer.
FIG. 2A shows a cross section view of an N.sup.- well pixel with a
PMOS transistor formed therein and a schematic view of an NMOS
transistor used to read the pixel.
FIG. 2B shows a schematic view of the circuit of FIG. 2A.
FIG. 3A shows a cross section view of an N.sup.- well pixel with a
PMOS transistor and an N.sup.+ region formed therein and a
schematic view of a two NMOS transistor circuit used to read the
pixel.
FIG. 3B shows a schematic view of the circuit of FIG. 3A.
FIG. 4A shows a cross section view of an N- well pixel with two
PMOS transistors formed therein and a schematic view of four NMOS
transistors used to read the pixel.
FIG. 4B shows a schematic view of the circuit of FIG. 4A.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Refer now to FIGS. 1 4B of the drawings for a description of the
preferred embodiments of this invention. FIG. 1 shows a top view of
a number of N.sup.- wells 12 formed of N.sup.- type silicon in a
P.sup.- type silicon substrate 10. Typically, but not necessarily,
the P.sup.- type silicon substrate 10 is a P.sup.- type epitaxial
silicon layer. FIG. 1 shows four N.sup.- wells 12 as an example
however the actual number will be larger or smaller, typically
smaller, arranged in an array. Each N.sup.- well 12 forms a PN
junction diode with the surrounding P.sup.- silicon material. The
N.sup.- wells 12 are biased such that the potential of the N.sup.-
wells 12 are higher than the P.sup.- silicon material 10 and the PN
junction is back biased. This back biased PN junction forms a pixel
which can accumulate carriers generated by an external signal to be
read during a readout period. Typically, but not necessarily, the
external signal is optical radiation and the carriers are
photo-generated carriers.
FIG. 2A shows a cross section view of one of the N.sup.- wells 12
and the surrounding P.sup.- silicon material 10. As shown in FIG.
2A, a first P.sup.+ type silicon region 14 and a second P.sup.+
type silicon region 16 are formed in the N.sup.- well 12. The first
P.sup.+ type silicon region 14 forms the source and the second
P.sup.+ type silicon region 16 forms the drain of a PMOS, P channel
metal oxide semiconductor, transistor 19. A gate oxide 18 is formed
over the channel 28 of the PMOS transistor 19. A gate electrode 20
is formed on the gate oxide 18. The N.sup.- well 12 is biased at
the highest potential in the circuit when the pixel is reset. This
will allow the N.sup.- well 12 region to collect all the
signal-generated electrons within a diffusion length of the N.sup.-
well 12 and P.sup.- substrate junction. Biasing the N.sup.- well 12
at the highest potential in the circuit during reset allows a
circuit tolerant to a 100% fill factor for the pixel. In this
example the highest potential in the circuit is the V.sub.DD
potential and is between about 4.5 and 5.5 volts, usually 5.0
volts. As shown in FIG. 2 the P.sup.- substrate is held at ground
potential by means of a P.sup.+ contact 21 into the P.sup.-
substrate which is held at ground potential. Either the first
P.sup.+ region 14 or the second P.sup.+ region 16 can be used to
reset the pixel by raising the potential of the selected P.sup.+
region to V.sub.DD while the pixel is being reset and then
returning the selected P.sup.+ region to ground potential while the
pixel is accumulating signal-generated electrons.
FIGS. 2A and 2B show a circuit for reading and resetting the pixel
using a single NMOS, N channel metal oxide semiconductor,
transistor 22 per pixel. Like reference numbers are used to denote
like circuit elements in FIGS. 2A and 2B. As shown in FIG. 2A, the
drain of the NMOS transistor 22 is connected to an output node 24
and the source of the NMOS transistor 22 is connected to the source
14 of the PMOS transistor 19. The drain 16 of the PMOS transistor
19 is connected to a reset node 30. The gate of the NMOS transistor
22 is connected to a select node 26. During pixel reset the NMOS
transistor 22 is turned off and the reset node 30 is raised from
ground potential to a potential of V.sub.DD to reset the pixel.
This sets the potential of the pixel to V.sub.DD V.sub.PB where
V.sub.PB is the potential drop across the junction between the
drain 16 of the PMOS transistor and the N.sup.- well. The NMOS
transistor is turned on and off by means of a potential applied to
the select node 26. After the reset of the pixel has been completed
the reset node 30 is returned to ground potential and the NMOS
transistor 22 remains turned off while the pixel accumulates
signal-generated carriers. Since the source 14 of the PMOS
transistor 19 is floating and the drain 16 of the PMOS transistor
19 is at ground potential during the charge accumulation period the
PMOS within the N.sup.- well 12 will not impact the collection of
the signal-generated carriers by the pixel. After the accumulation
period has been completed the NMOS transistor is turned on and the
charge accumulated by the pixel can be read by detecting the signal
at the output node 24.
Alternatively the drain 16 of the PMOS transistor 19 can be
permanently connected to ground potential by holding the reset node
30 at ground potential. This has the advantage of eliminating the
need for a separate reset line to be bussed to the pixel. In this
configuration during reset the NMOS transistor 22 is turned on and
the output node 24 is set to V.sub.DD. This brings the source 14 of
the PMOS transistor 19 to very nearly V.sub.DD potential thereby
resetting the pixel. After the pixel has been reset the NMOS
transistor is turned off while the pixel accumulates
signal-generated carriers. As before, since the source 14 of the
PMOS transistor 19 is floating and the drain 16 of the PMOS
transistor 19 is at ground potential during the charge accumulation
period, the PMOS within the N.sup.- well 12 will not impact the
collection of the signal-generated carriers by the pixel. After the
accumulation period has been completed the charge accumulated by
the pixel is read. One method of reading the pixel is to turn the
NMOS transistor on and detect the charge accumulated by the pixel
at the output node 24.
The potential of the N.sup.- well 12 and the floating PMOS source
16 will change based on the amount of signal-generated carriers
accumulated by the pixel during the charge accumulation period. For
readout of the accumulated charge the body effect can be utilized
to form a dual gate PMOS transistor 19 using the PMOS transistor 19
as a source follower. This is shown schematically in FIG. 2B
showing the NMOS transistor 22 having a source connected to the
output node 24 and the gate connected to a select node 26. The
reset node 30 is either connected to ground or used for resetting
the pixel. The N.sup.- well 12 forms a second gate for the dual
gate PMOS transistor 19 since the potential of the N.sup.- well 12
affects the conductivity of the channel 28 of the PMOS transistor
19, see FIG. 2A. The gate 20 of the PMOS transistor 19 can be used
as a gain control in this case.
There are several readout circuits that can be used with the pixel
with the embedded gate PMOS transistor 19 of this invention. FIGS.
3A and 3B show an example of one of these circuits. Like reference
numbers are used to denote like circuit elements in FIGS. 3A and
3B. In this example as in the previous example, as shown in FIG.
3A, a first P.sup.+ region 14 forms the source and a second P.sup.+
region 16 forms the drain of a PMOS transistor 19 formed in the
N.sup.- well 12. The N.sup.- well is formed in a P.sup.- substrate
10. A gate oxide 18 is formed over the channel 28 of the PMOS
transistor 19 and a gate electrode 20 is formed on the gate oxide
28. The drain 16 of the PMOS transistor 19 is connected to a reset
node 30, and the P.sup.- substrate 10 is held at ground potential
by means of a P.sup.+ contact 21 in the P.sup.- region 10. As in
the previous example, the source of a first NMOS transistor 22 is
connected to the source 14 of the PMOS transistor 19 and the drain
of the first NMOS transistor 22 is connected to an output node 24.
As shown in FIG. 3A an N.sup.+ 34 region is formed in the N.sup.-
well 12 and connected to the source of a second NMOS transistor 32.
The drain of the second NMOS transistor 32 is connected to the gate
of the first NMOS transistor. The gate of the second NMOS
transistor 32 is connected to the source of the first NMOS
transistor 22. The diode 31 in FIG. 3B represents the N.sup.+
region 34 and N.sup.- well junction 12 in FIG. 3A. The potential at
the cathode of the diode 31 is the potential of the N.sup.- well
and is the signal to be read after the pixel has completed a charge
accumulation cycle.
During the reset operation the gate 20 of the PMOS transistor 19 is
held at ground potential and the reset node 30 is held at V.sub.DD
potential. In this example V.sub.DD is the highest potential in the
circuit and is between about 4.5 and 5.5 volts, typically 5.0
volts. This turns the PMOS transistor 19 on, sets the N.sup.-
region 12 to a potential of nearly V.sub.DD, V.sub.DD minus a small
built in potential, and turns the second NMOS transistor 32 on.
This built in potential is the potential drop across the P.sup.+
source and N.sup.- well junction. This also turns first NMOS
transistor 22 off since the potential at the gate of the first NMOS
transistor 22 is less than the potential at the source of the first
NMOS transistor 22. The reset node 30 is then returned to V.sub.DD
potential turning the PMOS transistor 19 off to begin charge
integration. If the potential of the gate 20 of the PMOS transistor
19 is modulated the charge conversion gain can be varied. The
second NMOS transistor 32 remains on, because the forward bias
remains greater than the threshold voltage. The first NMOS
transistor 22 remains off because the potential at the gate of the
first NMOS transistor 22 remains less than the potential at the
source of the first NMOS transistor 22. Since the first NMOS
transistor 22 is off during the reset operation the potential of
the output node 24 does not matter.
After the pixel has been reset the signal-generated carriers will
reduce the potential of the N.sup.- well 12 and the floating source
14 of the PMOS transistor 19. When the pixel is read the potential
of the gate 20 of the PMOS transistor 19 is ramped from V.sub.DD to
ground potential. When the potential of this gate 20 becomes less
than the potential at the source 14 of the PMOS transistor 19 minus
the threshold voltage of the second NMOS transistor 32 the PMOS
transistor 19 turns on. This will pull the potential of the source
14 of the PMOS transistor 19 down to ground potential and reverse
bias the diode 31, see FIG. 3B. This causes the second NMOS
transistor 32 to turn off and the signal level, the potential of
the N.sup.- well 12, is stored at the gate of the first NMOS
transistor 22. During the readout cycle the gate 20 of the PMOS
transistor 19 can be used as a gain adjust control.
The ramping of the potential of the gate 20 of the PMOS transistor
19 can be used to detect the pixel signal level, the potential of
the N.sup.- well, and can also be used in conjunction with a timer
for a basic analog to digital converter. The timer is started at
the time the potential at the gate 20 of the PMOS transistor 19
begins to ramp from V.sub.DD toward ground potential. The time at
which the PMOS transistor turns on is a digital representation of
the signal detected by the pixel. This time can be stored for
future use. If the pixels are arranged in an array of rows and
columns with a global timer is at the bottom of each column, the
times at which the PMOS transistor in each pixel of a selected row
turns on stored gives a digital representation of the signal and
forms a basic analog to digital converter.
Since the potential of the N.sup.- well 12 is stored at the gate of
the first NMOS transistor 22 a snapshot imager with in pixel
storage can be realized with the addition of a third NMOS
transistor 90 with the gate of the third NMOS transistor 90
connected to a sequential row addressing circuit 91 and the source
of the third NMOS transistor 90 connected to the output node 24.
Since the gate of the first NMOS transistor 22 stores the potential
of the N.sup.- well 12 in a non destructive fashion, an array of
rows and columns of pixels can integrate for an identical time
duration and store individual pixel signals at the gate of the
first NMOS transistor 22 of each pixel in the array. Using the
third NMOS transistor 90 as a readout transistor having a gate
connected to a sequential row addressing circuit 91 each row can be
selectively read out through a single output using a raster
scan.
This basic circuit block can be repeated and used for on pixel
correlated double sampling, CDS. This embodiment is shown in FIGS.
4A and 4B. Like reference numbers are used to denote like circuit
elements in FIGS. 4A and 4B. FIGS. 4A and 4B show two dual gate
PMOS transistors in a single N.sup.- well. As shown in FIG. 4A, a
first P.sup.+ type silicon region 40, a second P.sup.+ type silicon
region 42, and a third P+ type silicon region 44 are formed in the
N.sup.- well 12. The first P.sup.+ type silicon region 40 forms the
source of a first PMOS transistor 56 and the third P.sup.+ type
silicon region 44 forms the source of a second PMOS transistor 60.
The second P.sup.+ region 42 forms the drain of both the first PMOS
transistor 56 and the second PMOS transistor 60. A first gate oxide
46 and first gate electrode 52 are formed over the channel of the
first PMOS transistor 56. A second gate oxide 48 and second gate
electrode 50 are formed over the channel of the second PMOS
transistor 60. As in previous embodiments, the N.sup.- well 12 is
biased at the highest potential in the circuit when the pixel is
reset. This will allow the N.sup.- well 12 region to collect all
the signal-generated electrons within a diffusion length of the
N.sup.- well 12 and P.sup.- substrate junction. In this example the
highest potential in the circuit is the V.sub.DD potential. In this
example V.sub.DD is between 4.5 and 5.5 volts, usually 5.0 volts.
As shown in FIG. 4A the P.sup.- substrate is held at ground
potential by means of a P.sup.+ contact 21 into the P.sup.-
substrate which is held at ground potential. The pixel is reset by
raising the potential of the reset node 58, connected to the second
P.sup.+ region 42, 58, to V.sub.DD while the pixel is being reset
and then returning the reset node 58 to ground potential while the
pixel is accumulating signal-generated electrons.
The second P.sup.+ region 42, which forms a common drain of the
first 56 and second 60 PMOS transistors, is connected to the reset
node 58, and the P.sup.- substrate 10 is held at ground potential
by means of a P.sup.+ contact 21 in the P.sup.- region 10. The
source of a first NMOS transistor 70 is connected to the source 40
of the first PMOS transistor 56 and the drain of the first NMOS
transistor 70 is connected to a first output node 78. As shown in
FIG. 4A a first N.sup.+ region 82 is formed in the N.sup.- well 12
and connected to the source of a second NMOS transistor 72. The
drain of the second NMOS transistor 72 is connected to the gate of
the first NMOS transistor 70. The gate of the second NMOS
transistor 72 is connected to the source of the first NMOS
transistor 70. The source of a third NMOS transistor 74 is
connected to the source 44 of the second PMOS transistor 60 and the
drain of the third NMOS transistor 74 is connected to a second
output node 80. As shown in FIG. 4A a second N.sup.+ region 84 is
formed in the N.sup.- well 12 and connected to the source of a
fourth NMOS transistor 76. The drain of the fourth NMOS transistor
76 is connected to the gate of the third NMOS transistor 74. The
gate of the fourth NMOS transistor 76 is connected to the source of
the third NMOS transistor 74.
FIG. 4B shows a schematic diagram of the circuit shown in FIG. 4A
for easier understanding of the operation of the circuit of FIGS.
4A and 4B. A first diode 83 in FIG. 4B represents the first N.sup.+
region 28 and N.sup.- well 12 junction in. FIG. 4A. A second diode
85 in FIG. 4B represents the second N.sup.+ region 84 and N.sup.-
well 12 junction in FIG. 4A. The potential at the cathodes of the
first diode 83 and second diode 85 is the potential of the N.sup.-
well and is the signal to be read after the pixel has completed a
charge accumulation cycle.
During the reset operation the potentials of the first gate 52 of
the first PMOS transistor 56 and the second gate 50 of the second
PMOS transistor 60 are set at ground potential while the potential
of the reset node 58 is raised from ground potential to V.sub.DD.
After the reset has been completed the potential at the second-
gate 50 is raised to V.sub.DD while potential of reset node 58
remains at V.sub.DD and the potential of the first gate 52 remains
at ground. This stores the reference voltage on the PN junction
between the N.sup.- well 12 and the P.sup.- substrate 10 the at the
gate of the third NMOS transistor 74. The potential of the reset
node 58 is then returned to ground potential with the potential at
the second gate 50 held at V.sub.DD and the charge integration
cycle begins. During the charge integration cycle the voltage
across the PN junction between the N.sup.- well and the P.sup.-
substrate decreases and the potential of the first gate 52
increases as charge is accumulated. At the end of the charge
integration cycle the potential of the second gate 50 is returned
to ground potential, the reset node 58 remains at ground potential
and the voltage across the PN junction between the N.sup.- well 12
and the P.sup.- substrate 10, from which the signal generated
charge can be determined, is stored at the gate of the first NMOS
transistor 70. The difference in potential between the second
output node 80 and the first output node 78 gives an image signal
with reduced noise and reduced pixel to pixel non-uniformity to
accomplish on pixel correlated double sampling, CDS.
As in the previous example, since the potentials at the gates of
the first 70 and third 74 NMOS transistors are stored in a non
destructive fashion a snapshot imager with in pixel storage can be
realized with the addition of a fifth NMOS transistor 92, with the
gate of the fifth NMOS transistor 92 connected to a sequential row
addressing circuit 93 and the source of the fifth NMOS transistor
92 connected to the first output node 78, and a sixth NMOS
transistor 94, with the gate of the sixth NMOS transistor 94
connected to a sequential row addressing circuit 95 and the source
of the sixth NMOS transistor 94 connected to the second output node
80, as shown in FIGS. 4A and 4B. As in the previous example, an
array of rows and columns of pixels can integrate for an identical
time duration and store individual pixel signals at the gates of
the first 70 and third 74 NMOS transistors of each pixel in the
array. Using the fifth 92 and sixth 94 NMOS transistors as readout
transistors having their gates connected to a sequential row
addressing circuits, 93 and 95, each row can be selectively read
out using a raster scan.
In this invention an N.sup.- well formed in a P.sup.- substrate is
used to form the junction for accumulating signal generated
carriers. Those skilled in the art will readily recognize that the
invention will work equally well using a P.sup.- well in an N.sup.-
substrate. In this case P.sup.+ regions are replaced by N.sup.+
regions, N.sup.+ regions are replaced by P.sup.+ regions, P.sup.-
regions are replaced by N.sup.- regions, N.sup.- regions are
replaced by P.sup.- regions, P regions are replaced by N regions, N
regions are replaced by P regions, PMOS transistors are replaced by
NNOS transistors, NMOS transistors are replaced by PMOS
transistors, and the highest voltage in the circuit is replaced by
the lowest voltage in the circuit.
While the invention has been particularly shown and described with
reference to the preferred embodiments thereof, it will be
understood by those skilled in the art that various changes in form
and details may be made without departing from the spirit and scope
of the invention.
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